Slab-on-ground foundation design method

ABSTRACT

This disclosure describes the invention of an improved WRI slab-on-ground foundation design method. The improved rational design method integrates any combination of rebar, wire mesh, and post-tensioned cable reinforcements into the foundation design. According to the method, cantilever lengths are derived for at least one rectangular section of a foundation for a structure. Foundation design loads typically consist of uniformly distributed loads. The invention adds the capability to include line loads representing loads transmitted to the foundation. Foundation design quantities such as moments, shears, and deflections are calculated from uniformly distributed loads. Moments, shears, and deflections from line loads are added to these design quantities. A deflection multiplier for the foundation deflections from line loads is employed for compatibility with WRI design procedures. A cracking potential of a foundation was formulated as a function of tensile stresses.

FIELD OF THE INVENTION

The present invention relates generally to the field of building residences and low-rise structures, and more particularly to a slab-on-ground foundation design method.

BACKGROUND

Slab-on-ground foundations are widely used for the construction of residential and low-rise commercial structures, as well as in other structural applications. Ribs (also called beams) usually form an integral part with the slab. For the purposes of this description we have assumed that the Slab-on-Ground foundations may or may not include beams. The design method applies to both Slab-on-Ground foundation designs with or without ribs. Soil deformations create stresses on a foundation as the soil expands and contracts, often in response to variation in moisture content of the soil.

Given variations in soil type and conditions across even a relatively small foundation area, the foundation may be subject to heave or settlement conditions wherein a portion of the foundation is left unsupported as soil underlying the foundation swells, shrinks or settles, resulting in a condition of center lift, where the center area of the foundation is lifted relative to the foundation edges, or edge lift, where the edges of the foundation are lifted relative to the center. Highly expansive soils, such as expansive clay soils, exhibit expansion and contraction with changes in soil moisture variation to a much greater degree than other soil types.

Expansion and contraction of soil beneath a foundation, especially to the degree associated with highly expansive soil types, create problems for foundations, and for a building supported by the foundation. As the soil expands or contracts beneath the foundation, the foundation may be subjected to great stresses that can result in cracking, or even breaking of the foundation. Cracking and deformation of the foundation result in shifting of the building atop the foundation, and may result in serious interior and exterior wall cracking and possible structural compromise or distress.

Various methods have been employed to strengthen concrete slab-on-ground foundations to limit deflections and to reduce or eliminate foundation cracking. Current building codes generally allow for several methods for the design of slab-on-ground foundations on expansive soils. One method employs steel reinforcement (wire mesh and/or rebar) within the foundation. In another method, post-tensioned reinforcement tendons are located within the foundation for reinforcement.

A design method adopted by the Wire Reinforcement Institute (WRI) uses steel rebar and wire mesh for reinforcement of slab-on-ground foundations with beams. The WRI design method is used to determine, among other design attributes, the number, size and spacing of beams formed in the concrete slab, and amount of reinforcing material to include within the foundation.

Design requirements are determined in view of the quality and character of the soil on which the foundation is to be built. It can be recognized that a foundation on a highly expansive soil will need to be designed for greater strength than a foundation placed on a soil that is not expansive, since expansion and contraction of the soil may cause that portions of the foundation on expansive soil may be relatively more or less supported than other portions, stressing the foundation.

According to the WRI design method, the soil characteristics and qualities of the soil underlying the foundation are quantified by an equivalent plasticity index. Because a regional climate affects the stability of the moisture content of the foundation soil, a climate rating expressed as a coefficient is considered along with the plasticity index, slope of the natural ground and the size of the slab to determine the soil bearing and deformation characteristics.

As a measure of the stability of the soil and the soil's ability to uniformly support a foundation, the climatic rating coefficient and the plasticity index is indicative of possible deformations of the foundation site due to swelling and shrinking of the soil. Thus, the climatic rating coefficient and the plasticity index and other factors such as the size of the foundation, the slope of the natural ground and the unconfined compressive strength of the soil may be used to determine cantilever lengths for the foundation on which the method relies to derive design quantities. Cantilever lengths are determined for both the longitudinal and lateral directions of the foundation.

If a foundation slab has an irregular shape, the shape of the slab is divided into overlapping rectangular sections that represent the foundation shape. Design calculations are performed for each individual rectangle, similar to the WRI method. Cantilever lengths may be determined along the longitudinal and lateral directions of each rectangle of the foundation. Additionally, spacing of beams in the foundation in longitudinal and lateral, or transverse, directions may be determined for each rectangular region, based primarily on the cantilever length for a given design. Beam spacing decreases as the cantilever length increases, reflecting the greater unsupported areas of the foundation.

The WRI method relies on a uniformly distributed loading model, thus all loading is modeled as a uniformly distributed load, reducing the entire foundation and structural weight and design loads to a uniformly distributed loading per unit area of the foundation area. From the cantilever lengths, along with expected or design foundation loading, the maximum bending moment, shear force and deflection may be calculated for each of the longitudinal and lateral directions of the foundation according to this uniformly distributed loading model. The moments, shear forces, and deflections may be used to determine the amount of steel rebar and wire mesh for reinforcements to be included in a foundation design, or may be used to evaluate the suitability of a particular foundation design for the demands of a particular site.

Because the WRI method relies on a uniformly distributed loading model to represent all loads, it is not appropriate for all types of residences and low-rise structures. For certain structures, for example a residence with masonry walls and a truss roof, line loads that simulate the weight of the walls and the loads the walls carry along the edge of the structure may be significant and may not be adequately represented or accounted for by the uniformly distributed loading assumption of the WRI method.

Another design method is promoted by the Post-Tensioning Institute (PTI). The PTI method produces a design for a foundation using post-tensioned cables for reinforcement and sometimes rebar for local reinforcement. The PTI method design formulas consider perimeter line loads on the foundation, as well as a uniformly distributed loading model.

SUMMARY

The slab-on-ground foundation design method improves, over the conventional WRI design methods, the design flexibility and accuracy of the WRI method by considering line loads in conjunction with averaged, or uniformly distributed loading in the calculation of design moments, shears, and deflections for a foundation.

According to the method of the present invention, a cantilever length is determined for a given foundation according to generally known design procedures.

A uniformly distributed design foundation loading representing the weight of the foundation plus the uniformly distributed design loads on the foundation as well as appropriate design line loads representing the weight of the walls and the design loads they carry are determined for the foundation loading of a structure.

Design moments, shears, and deflections are calculated using generally known design procedures and principles, such as those introduced by the WRI design method, for the uniformly distributed foundation design loads.

Corrections are then applied to the design moments, shears, and deflections based on the design line load contributions for foundation support conditions such as those assumed in the WRI design procedure. The corrected design moments, shears, and deflections are used to verify the assumed preliminary design parameters including the number, width, and depth of beams for the foundation, and the type(s) and amount(s) of the reinforcing materials as well as an assessment of the cracking potential of the foundation design. Further correction of the design moments for cable eccentricity may be necessary if post-tensioned cable reinforcements were included in the preliminary foundation design.

The corrected design moments, shears, and deflections are used to determine any combination of rebar, wire mesh, and post-tensioned cable reinforcements to be included in the foundation.

In addition to producing a new foundation design, the method may be used to evaluate, and to optimize, an existing foundation design.

These and other features, aspects, and advantages of the present invention will become better understood with regard to the following description, appended claims, and accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flowchart describing a slab-on-ground foundation design method according to the present invention.

FIG. 2A is a loading diagram illustrating a center lift condition for a slab-on-ground concrete foundation for a uniformly distributed load.

FIG. 2B is a loading diagram illustrating an edge loading condition for a slab-of an on-ground concrete foundation in a center lift condition.

FIG. 3A is a loading diagram illustrating an edge lift condition for a slab-on-ground concrete foundation for a uniformly distributed load.

FIG. 3B is a loading diagram illustrating a center loading condition for a slab-on-ground concrete foundation in an edge lift condition.

FIG. 4 illustrates a cross section of a typical foundation design according to the slab-on-ground foundation design method of the present invention.

FIG. 5 is a block diagram of a computer system for a computerized implementation of a slab-on-ground foundation design method of the present invention.

FIG. 6 is a screen image of a data entry display for entering geotechnical site information in a computer program for performing a slab-on-ground foundation design method of the present invention.

FIG. 7 shows the screen image of FIG. 6 after interpretation of the geotechnical data.

FIG. 8 is a screen image of a data entry display for the entry of additional data relating to the foundation design.

FIG. 9 is a screen image showing a display of the completed foundation cantilever length calculations and related data.

FIG. 10 is a screen image showing a display of design properties according to the slab-on-ground foundation design method of the present invention.

DETAILED DESCRIPTION OF VARIOUS EMBODIMENTS

A slab-on-ground foundation design method according to the present invention encompasses the rational use of any combination of steel reinforcement (wire mesh and/or rebar) and post-tensioned cable reinforcement in a concrete slab-on-ground foundation with or without ribs or beams, and particularly for such foundations on expansive soils which is unique. The slab-on-ground foundation design method is based on and extends well known design procedures according to the Wire Reinforcement Institute (WRI), referred generally to as the “WRI method (updated)”, or simply the “WRI method”, to incorporate other combinations of reinforcements such as post-tensioned cable reinforcement by itself, steel reinforcement (wire mesh and/or rebar) by itself, or a mix of post-tensioned cable and steel reinforcements.

By comparison, the Post-Tensioning Institute (PTI), generally referred to as the “PTI method” mentions the use of additional rebar of a certain minimum length that starts at the edge of the slab and has a certain minimum length for shrinkage crack control. The length of this rebar has no relation to the length or the width of the foundation. In contrast to the present invention, the PTI method does not integrate rebar and post-tensioning cable in its design procedure for the main reinforcement of the foundation. Neither is the PTI method capable of using different values for line loads in the long and the short direction of the foundation (“perimeter loads”), and it does not include center line loads.

The foundation design method according to the present invention improves design flexibility and accuracy by introducing line loads (around the perimeter and in the center of the slab in both longitudinal and lateral directions) rather than averaged loads (according to the WRI method) or perimeter and averaged loads (according to the PTI method) alone.

Additionally, the foundation design method also includes a procedure to determine the cracking potential of a foundation. By rating a foundation's cracking potential as low, medium, or high, various designs may be compared for performance.

Referring to FIG. 1, the slab-on-ground foundation design method is generally described. Initially,. geotechnical data, including soil parameters and other variables, is determined (at step 102) from which the equivalent plasticity index (PI_(e)) for a construction site where the foundation will be placed is calculated, according to the WRI procedures. For judgment as to the suitability of a particular design for its intended location, the so-called “Potential Vertical Rise” (PVR) may also be derived.

A preliminary foundation design is prepared (at step 103), including at least the foundation shape and dimensions, and foundation and structural weights. The preliminary design defines weight and loading aspects of the structure, including line loads. The line loads may represent loads due to exterior walls of the structure, and loads due to interior walls of the structure as well as the loads these walls carry. The preliminary design also includes beam spacing and width, amounts and placement of steel reinforcement, and amounts and placement of post-tensioned cable reinforcement. Often, aspects of the preliminary design, such as the beam width and spacing, amounts and placement of steel reinforcement, amounts and placement of post-tensioned cable reinforcement, and combinations of steel reinforcement and post-tensioned cable reinforcement, are determined by architectural and engineering assumptions, estimates, and “best guesses.”

Following WRI procedures, cantilever lengths L_(c) for the foundation are calculated from the equivalent plasticity index PI_(e) (at step 104) of the soil and other geotechnical variables. Alternatively, the application of other soil mechanics and geotechnical theories may be used to determine cantilever length or to apply corrections to the computed cantilever length. At least one cantilever length is determined, and generally a cantilever length will be determined for both the longitudinal and lateral dimensions of each rectangular section of the foundation according to the preliminary design.

Design moments, shears, and deflections of the foundation are calculated based on the cantilever length L_(c), the unit weight and the uniformly distributed loading of the foundation, and on line loadings. The uniformly distributed loads and line loadings are calculated (step 106) based on the preliminary design.

To determine if loads on the foundation soil will be within, or will exceed, the soil bearing capacity (at step 108), the “all-in weight” of the foundation, the structure, and design loads is determined to calculate soil bearing stresses.

It is determined if the loads on the foundation soil exceed the soil bearing capacity. If the soil bearing capacity is exceeded, the preliminary design may be modified accordingly, returning to step 103.

Foundation moments, shears and deflections are calculated, first generally according to the WRI procedures using uniformly distributed foundation weight and uniformly distributed design loads on the foundation (step 110). The moments, shears, and deflections are determined for both center lift and edge lift conditions in longitudinal and lateral directions of the foundation. The moments, shears, and deflections (from step 110) are then corrected by the contributions of the line loads (step 112) to obtain the corrected foundation design moments loads, shears, and deflections.

Calculation of design moments, shears, and deflections for a center lift condition for uniformly distributed loads, is generally according to the WRI method for that part of the foundation loading represented by the weight of the foundation and the uniform design loads placed on it (with reference to the center lift loading diagram of FIG. 2A).

The symbols in the figures have the following meaning:

-   -   L=Length of the slab in bending direction     -   L′=Width of the slab perpendicular to the bending direction     -   L_(c)=Cantilever length     -   w=uniform slab loading (psi) from slab weight and live loads         (without line load contributions)     -   P_(e)=Edge line load (plf) representing the edge wall load as         well as the loads the wall carries.     -   P_(c)=Center line load (plf) representing the center wall load         as well as the loads the wall carries.

For the center lift condition, edge line loads P_(e) can be introduced. The edge line loads are only applied conservatively to center lift conditions. No centerline loads are applied to edge lift conditions because these loads will be transferred to the foundation in the area where the foundation is supported without an appreciable effect on the foundation slab elsewhere. A loading diagram of an edge line loading condition is shown in FIG. 2B. Moment, shear, and deflection for the center lift condition, including line loads, may be calculated using generally accepted conventional formulations.

The maximum values of the corresponding quantities from the edge loads and the uniformly distributed loads are linearly added to obtain design values representing a conservative solution for the combined loads.

When cables are used to post tension the concrete, the design values for moment, shear and deflection may need further correction. An additional moment caused by their force and their eccentricity is introduced. The additional moment and maximum deflection caused by the eccentricity of the pretension is formulated as follows:

The moment contribution M of the resultant post tensioned cable force F_(c) (of all post tensioned cables combined) with eccentricity E_(cc) is computed using conventional procedures. It is used to compute the additional deflection caused by the cable forces.

The moment contribution and the additional deflection values caused by the cable forces is assumed constant over the length of the slab. The local influence of cable drop near the foundation edges is ignored. Its dependence on the deflected shape of the slab is ignored, implying there are no shear contributions from deflections or cable drops considered.

The so-called P-Delta (P-Δ) effects are ignored. The moment contribution and the additional deflection values caused by the eccentricity of the cable forces are linearly added to the corresponding maximum values from edge loads and uniform loads on the foundation to obtain conservative design values for the foundation for the center lift condition.

The effect on the deflection of the eccentricity of the cable forces are usually insignificant.

Edge lift conditions are also considered in the design. Calculation of design moments, shears, and deflections for an edge lift condition, is generally according to the WRI method for that part of the foundation loading represented by the weight of the foundation and the design loads placed on it (with reference to the edge lift loading diagram of FIG. 3A).

Center line loads P_(c) are only considered as additional loads for edge lift. No edge line loads P_(e) are generally considered in the case of edge lift. It is conservatively assumed that these loads will be directly transferred to the foundation without an appreciable effect on the foundation slab. A loading diagram of the centerline P_(c) loads is shown in FIG. 3B. Moment, shear, and deflection for the edge lift condition, including center line loads, may be calculated using conventional and well known procedures.

We again assumed that the cantilever length L_(c) does not change for the edge lift condition. This implies that the soil support loads of the slab are independent of the load distribution and the deflected shape of the slab.

Calculation of design moments, shears, and deflections for the edge lift condition for uniformly distributed loads, is generally according to the WRI method for that part of the foundation loading represented by the weight of the foundation and the uniform design loads placed on it (with reference to the edge lift loading diagram of FIG. 3A ). Maximum values of the corresponding quantities from the center line loads are added linearly to obtain a conservative solution for the combined uniformly distributed foundation loads and center line loads.

As with the center lift design formulae, the effect of post tension cable eccentricity on the design quantities may be ignored as insignificant, however we have included this effect in our application using generally accepted procedures.

The so-called P-Delta effects are ignored. The moment contribution and the additional deflection values caused by the eccentricity of the cable forces are linearly added to the corresponding maximum values from edge loads and uniform loads on the foundation to obtain conservative design values for the foundation for the edge lift condition. The effect on the deflection of the eccentricity of the cable forces are usually insignificant.

In the calculation of the corrections to the deflection for center lift and edge lift conditions and cable eccentricity, the same deflection multipliers that are built into the WRI deflection formulas are used for consistency although technical evaluations could justify the use of a different multiplier.

In those cases where the deflection and/or the resultant post tensioned cable forces are unusually large, their influence on the design quantities could become significant for both center lift and edge lift conditions. In that case, formulas for moment, shear and deflection may be rewritten using well known and conventional principles for the influence so-called P-Delta effect, in which case the design values for moment, shear and deflection are obtained through conventional and well known iterative procedures.

Based on the computed design moments, shears, and deflections, the foundation design may be completed, according to well known design processes such as the ultimate strength method. If only post-tensioned cable is applied as reinforcement, the rate of elongation of concrete is limited to a pre-determined value in the calculation of moment capacity according to said ultimate strength method. The computed design moments, shears, and deflections may be used to evaluate the preliminary design, either confirming that the preliminary design is sufficient for the design loads or discovering shortcomings (step 114). Alternatively, the design moments, shears, and deflections may be used to update design parameters, such as the width and spacing of beams, amounts and placement of steel reinforcement, and amounts and placement of post-tensioned cable reinforcement.

If modification to the preliminary design is desired or required (at step 116), such as in case of unsatisfactory results discovered in the evaluation of the preliminary design or in order to improve or optimize aspects of the preliminary foundation design, the preliminary foundation design may be revised, returning to step 113 and repeating the procedure to optimize strength, flexibility, cost, and other factors. For example, tradeoffs among design parameters such as beam spacing, steel reinforcement, and post-tensioned cable reinforcements can be contemplated.

A typical partial cross section of a ribbed foundation designed according to the slab-on-ground foundation design method including both rebar and post-tensioned cables in a beamed foundation, is shown in FIG. 4.

A foundation design may be evaluated for its cracking potential by evaluating the computed extreme tensile fiber stress f_(t) caused by foundation loading (weight and loads) and support. f_(t) is calculated, assuming that the concrete responds perfectly elastically, using standard and recognized design procedures. A low, medium, or high cracking potential is determined, based on the calculated f_(t), as follows: Low: f _(t) <C _(low) √{square root over (f′_(c))}  (1) Medium: C _(low) √{square root over (f′_(c))} ≦f _(t) ≦C _(high) √{square root over (f′_(c))}  (2) High: f _(t) >C _(high) √{square root over (f′_(c))}  (3)

Where: f′_(c) is the specified compressive strength of concrete, C_(low) is a first cracking potential coefficient defining a lower threshold, and C_(high) is a second cracking potential coefficient defining an upper threshold.

A C_(low) of about 7.5, and a C_(high) of about 12 are exemplary to define the lower and upper threshold for certain types of concrete, although different values may be selected for different types of concrete or based on experience. The cracking potential may be determined for both the top of the slab (or the top of the beams) and the bottom of the beams.

The slab-on-ground foundation design method is now described in greater detail in reference to a computer program implementation of the slab-on-ground foundation design method. A computer program implements the slab-on-ground foundation design method by providing user interfaces for entry of data, performing computations according to the method, and presenting resulting data to the user. The computer program provides facilities for generating a foundation design according to calculated design moments, shears, and deflections, and for evaluating and modifying an initial design using the calculated design moments, shears, and deflections.

The computer program is loaded in, and executed by, a computer such as the general purpose computer system 500 illustrated in FIG. 5. According to a typical computer architecture, the computer system 500 comprises a microprocessor 501 connected by a bus 503 to an area of main memory 502, comprising both random access memory (RAM) 504, and read only memory (ROM) 506, and a storage device 508 such as a disk storage device having means for reading a coded set of program instructions on a computer readable medium which may be loaded into main memory 502 and executed by the microprocessor 501. Additional storage devices may be provided, such as a media reader 510 for reading a removable storage media 512 such as a removable disk, CD-ROM, or the like. The computer program may be supplied as a computer program product comprising computer programming instructions stored on a computer readable medium such as the removable storage media 512.

The computer system 500 may include user interface components, such as a keyboard 514 or other data entry devices, and a display 516 or other data display or output devices. A printer 518 may be attached to the computer system 500.

While the computer program, and the computer system 500, are described in terms of a stand-alone computer environment, it can be understood that the computer program may be adapted to function in an Internet, mobile, or another environment. In such an alternate environment, the computer system 500 may include a communication interface 520 for communicating with a remote computer system or device, an Internet Service Provider (ISP), or another computing system or entity, such as over a computer network or the Internet 522.

The computer program allows for entry of data for the creation of a new foundation design. Data, and calculated design parameters, may be saved, and later retrieved for modification or reuse as the basis for a new design.

For a new foundation design, site information including soil and environmental parameters to be used in the design process must be entered. Referring to FIG. 6, a data entry screen display is shown for the entry of soil parameters. The soil parameters include the soil plasticity index PI for at least one, and preferably several, layers of soil for a reasonable depth beneath the foundation. For example, a plasticity index may be determined at every foot for the underlying fifteen (15) feet of soil. The various soil layers may be assigned a weighting based on depth. The plasticity indices are used together to compute the effective plasticity index PI, which is simply a weighted average of the PI of each of the layers to a depth of 15 feet. Corrections to the value of the PI are applied according to the WRI method to obtain the equivalent plasticity index PI_(e). In addition, another method to evaluate the swell potential, the so-called potential vertical rise (PVR) may be calculated according to any of several diverse theories and methods.

Turning to FIG. 7, the data entry screen display 600 is shown now with completed calculations based on the entered soil parameters. The entered, and computed, data may be evaluated for conformity to reasonableness expectations, and data that appears incorrect or suspect may be flagged or highlighted for verification or correction.

In addition to entry of the soil parameters, basic design data is entered for the foundation. Referring to FIG. 8, a data entry screen display 800 is shown for the entry of initial foundation and material parameters, including physical dimensions of the foundation (expressed as the dimensions of overlapping rectangular regions of the foundation if the foundation has an irregular plan shape), and loading factors such as total line loadings, design live loads, as well as material characteristics including concrete strength and weight, characteristics of reinforcing materials. Material costs may also be entered, as the cost of any given foundation design is a significant consideration when evaluating alternate designs.

Turning to FIG. 9, the cantilever length L_(c) may be calculated once the relevant data has been entered. Additional data may be used as adjustments to L_(c), such as ground slope correction, and a consolidation correction coefficient. These values are used to modify the L_(c), producing an effective cantilever length L_(ce). Additional modifications, or limitations to data such as minimum or maximum values for L_(c), minimum or maximum values for beam length, or additional modifiers as specified by relevant local code or practice recommendations and requirements, may be applied by the computer program to the computation of L_(c) or L_(ce). New values for L_(c) or L_(ce) may be calculated and displayed as data, corrections, or modification values are entered.

Along with the cantilever length values, estimated (or initial) values may be calculated for beam spacing and a measure for the required reinforcement (A_(s)F_(y)).

After all data has been input, including the soil parameters and basic design properties, design information such as moments of inertia, location of the neutral axis, and the like, can be computed. The cantilever lengths can be determined and the design values for moments, shears, and deflections, as well as the cracking potential can be computed. It is also confirmed that the soil bearing capacity is within tolerable limits. Foundation cost estimates can also be included based on unit prices for materials and the amount of materials in the foundation.

Referring to FIG. 10, a screen display 1000 is shown displaying results of the design calculations according to the present method. Modifiable data display fields 1002 are included to allow alteration of key design attributes or parameters, such as beam width and depth, amount of reinforcing materials, spacing of rebar in the slab and the post-tensioning stress level applied to the foundation. Upon alteration of design attributes or parameters, design calculations are repeated and the displayed data is updated. Values that, as a result of initial or updated design assumptions, attributes, and parameters, are insufficient to meet design requirements for the foundation may be flagged or highlighted.

It will be understood that the above-described embodiments of the invention are illustrative in nature, and that modifications thereof may occur made by those skilled in the art. Accordingly, this invention is not to be regarded as limited to the embodiments disclosed herein, but is to be limited only as defined in the appended claims. 

1. A slab-on-ground foundation design method, comprising the steps of: specifying a preliminary design for a foundation, the preliminary design defining at least a shape and dimensions of said foundation and at least one reinforcing element selected from a group consisting of rebar, and post-tensioned cable and design loads on said foundation including design line loads due to a structure to be built on said foundation; identifying soil conditions for a given construction site; deriving, based on said soil conditions, at least one cantilever length for at least one rectangular section of said foundation; calculating a uniformly distributed unit weight for said foundation as well as its design loads based on said preliminary design; calculating an all-in structural weight including said design loads of said structure; calculating design moments, shears, and deflections based on said cantilever lengths, said uniformly distributed unit weight of the foundation, and said design loads on the foundation; applying corrections to said design moments, shears, and deflections based on said design line loads and any loads attributable to post-tensioned cable to obtain corrected design moments, shears, and deflections for all loads on the foundation; and evaluating said preliminary design in view of said design moments, shears, and deflections and said corrected design moments, shears, and deflections.
 2. The method of claim 1, wherein, in applying corrections to said deflections, a deflection multiplier is used for compatibility with WRI deflection formulas.
 3. The method of claim 1, wherein said line loads represent loads transferred to said foundation through exterior walls of said structure.
 4. The method of claim 1, wherein said line loads represent loads transferred to said foundation though interior walls of said structure.
 5. The method of claim 1, further comprising the step of calculating foundation bearing loads based on said all-in structural weight and said cantilever lengths that are used to evaluate if the soil bearing capacity is adequate to carry the vertical loads.
 6. The method of claim 1, further comprising the step of using said design moments, shears, and deflections, and said corrected design moments, shears, and deflections, to determine design properties for a slab-on-ground foundation.
 7. The method of claim 6, wherein said design properties are used to evaluate a number and width of beams of said preliminary foundation design.
 8. The method of claim 6, wherein said design properties are used to evaluate an amount of steel reinforcement of said preliminary foundation design.
 9. The method of claim 6, wherein said design properties are used to evaluate an amount of post-tensioned cable reinforcement of said preliminary foundation design.
 10. The method of claim 6, wherein said design properties are used to evaluate an amount of both steel reinforcement and post-tensioned cable reinforcement of said preliminary foundation design.
 11. The method of claim 1, further comprising the step of evaluating any combination of steel reinforcement elements and post-tensioned cable reinforcement elements in a slab-on-ground foundation design to satisfy strength and deflection requirements compatible with said design moments, shears, and deflections, and said corrected design moments, shears, and deflections.
 12. A computer program product comprising a computer-usable medium having computer-readable program code embodied thereon, the computer-readable program code including computer instructions for: inputting preliminary design properties of a preliminary design for a foundation, the preliminary design defining at least a shape and dimensions of said foundation and design loads on said foundation including line loads due to a structure to be built on said foundation; identifying soil conditions for a given construction site; calculating, based on said soil conditions, and displaying at least one cantilever length for at least one rectangular section of said foundation; calculating a uniformly distributed unit weight for said foundation based on said preliminary design; calculating an all-in structural weight including said design loads of said structure; calculating and displaying design moments, shears, and deflections based on said cantilever length, said uniformly distributed unit weight of the foundation, and said design loads on the foundation; applying corrections to said design moments, shears, and deflections based on said design line loads to obtain corrected design moments, shears, and deflections for all loads on the foundation; and displaying said corrected design moments, shears, and deflections.
 13. The computer program product of claim 12, wherein said computer-readable program code further comprises computer instructions for deriving an equivalent Plasticity Index from said soil conditions.
 14. The computer program product of claim 12, wherein said computer-readable program code further comprises computer instructions for deriving a Potential Vertical Rise from said soil conditions.
 15. The computer program product of claim 12, wherein said computer-readable program code further comprises computer instructions for: computing a total load on soils supporting said foundation and said structure from a combination of at least the weight of the structure including its foundation and said design loads to compute soil bearing stresses; and displaying factors of safety for said bearing stresses with respect to the soil bearing capacity.
 16. The computer program product of claim 12, wherein said line loads include loads on said foundation due to exterior walls and loads supported by the exterior walls of said structure.
 17. The computer program product of claim 12, wherein said line loads include loads on said foundation due to interior walls and loads supported by the interior walls of said structure.
 18. The computer program product of claim 12, wherein said computer-readable program code further comprises computer instructions for using said design moments, shears, and deflections, and said corrected design moments, shears, and deflections, to evaluate the design properties of the preliminary (ribbed) slab-on-ground concrete foundation design.
 19. The computer program product of claim 18, wherein said preliminary design properties specify a number and width of beams in said foundation.
 20. The computer program product of claim 18, wherein said preliminary design properties specify an amount of steel reinforcement in said foundation.
 21. The computer program product of claim 18, wherein said preliminary design properties specify an amount of post-tensioned cable reinforcement in said foundation.
 22. The computer program product of claim 18, wherein said design properties describe a foundation design that includes at least one reinforcement element selected from a group consisting of steel reinforcement elements and post-tensioned cable reinforcement elements.
 23. The method of claim 12, wherein said computer-readable program code further comprises computer instructions for applying a combination of at least one reinforcement element, selected from a group consisting of steel reinforcement elements and post-tensioned cable reinforcement elements, in a (ribbed) slab-on-ground foundation design to indicate that strength and ductility and deflection requirements according to said design moments, shears, and deflections are satisfied.
 24. The method of claim 12, wherein said computer-readable program code further comprises computer instructions for using said design moments, shears, and deflections to evaluate said preliminary foundation design.
 25. A method of evaluating the cracking potential of a concrete slab-on-ground foundation, comprising the steps of: computing an extreme tensile fiber stress caused by loading and support of said foundation; computing at least one threshold as a function of the compressive strength of concrete (f′_(c)); comparing the extreme tensile fiber stress to said least one threshold (f′_(c)).
 26. The method of claim 25, wherein said at least one threshold includes a lower threshold and an upper threshold.
 27. The method of claim 26, wherein said cracking potential is determined to be low if said extreme tensile fiber stress is less than said lower threshold.
 28. The method of claim 26, wherein said cracking potential is determined to be medium if said extreme tensile fiber stress is between said lower and upper thresholds.
 29. The method of claim 26, wherein said cracking potential is determined to be high if said extreme tensile fiber stress is greater than said upper threshold. 